=== AP Decoding

With the QRA64 decoder Nico Palermo, IV3NWV, introduced a technique
for decoding with the aid of information that naturally accumulates
during a minimal QSO.  This _a priori_ (AP) information can be
used to increase the sensitivity of the decoder.

When an operator decides to answer a CQ, he already knows his own
callsign and that of his potential QSO partner.  He therefore knows
what to expect for at least 56 of the 72 message bits in a
standard-format response to his call.  The _WSJT-X_ decoders for QRA64
and FT8 can use these AP bits to decode messages containing them with
higher sensitivity than otherwise possible.

We have implemented AP decoding in slightly different ways in QRA64
and FT8.  To provide some explicit examples for users, we provide here
a brief description of the FT8 behavior.

The FT8 decoder always tries first to decode a signal without using
any AP information. If this attempt fails, and if *Enable AP* is
checked on the *Decode* menu, a second attempt hypothesizes that the
message contains callsigns MyCall and DxCall. If the QSO has
progressed to the point where signal reports have been exchanged, a
third attempt hypothesizes that the message contains the known
callsigns followed by RRR, RR73, or 73.

AP decoding attempts effectively set the AP bits to the hypothesized
values, as if they had been received perfectly.  The decoder then
proceeds to determine whether the remaining message and parity bits
are consistent with the hypothesized AP bits.  If a codeword is found
that the decoder judges to have high (but not overwhelmingly high)
probability of being correct, a ? character is appended when the
decoded message is displayed. Decodes thus marked are not sent to
{pskreporter} to avoid occasional misleading spots of false decodes.

Successful AP decodes are always labeled with an end-of-line indicator
of the form aP, where P is one of the single-digit AP decoding types
listed in Table 1.  For example, an a2 designator says that the
successful decode used MyCall as hypothetically known information.

[[AP_INFO_TABLE]]
.AP information types
[width="35%",cols="h10,<m20",frame=topbot,options="header"]
|===============================================
|P | Message components
|1 | CQ   &#160; &#160;   ?   &#160; &#160;   ? 
|2 | MyCall &#160; &#160; ?   &#160; &#160;   ? 
|3 | MyCall DxCall &#160; &#160;  ? 
|4 | MyCall DxCall RRR
|5 | MyCall DxCall 73
|6 | MyCall DxCall RR73
|===============================================

=== Decoded Lines

Displayed information accompanying decoded messages generally includes UTC,
signal-to-noise ratio in dB, time offset DT in seconds, and
audio frequency in Hz.  Some modes include additional information such
as frequency offset from nominal (DF), frequency drift (Drift or F1),
or distance (km or mi).

There may also be some cryptic characters with special meanings
summarized in the following Table:

[[DECODED_LINES_TABLE]]
.Notations used on decoded text lines
[width="50%",cols="h,3*^",frame=topbot,options="header"]
|===========================================
|Mode    |Mode character|Sync character|End of line information
|FT8     | ~            |      | ? &#160; aP
|JT4     | $            | *, # | f, fN, dNC
|JT9     | @            |      |
|JT65    | #            |      |
|JT65 VHF| #            | *, # | f, fN, dNC
|QRA64   | :            | *    | R
|ISCAT   |              | *    | M  N  C  T
|MSK144  | &            |      | N
|===========================================
Sync character::
 `*` - Normal sync +
 `#` - Alternate sync

End of line information::
 `?` - Decoded with lower confidence +
 `a` - Decoded with aid of some a priori (AP) information +
 `C` - Confidence indicator [ISCAT and Deep Search; (0-9,*)] +
 `d` - Deep Search algorithm +
 `f` - Franke-Taylor or Fano algorithm +
 `M` - Message length (characters) +
 `N` - Number of Rx intervals or frames averaged +
 `P` - Number indicating type of AP information (Table 1, above) +
 `R` - Return code from QRA64 decoder +
 `T` - Length of analyzed region (s)

